Comparison of Somatic and Autonomic Systems

Autonomic Nervous System (ANS)

The ANS consists of motor neurons that:

Innervate smooth and cardiac muscle and glands

Make adjustments to ensure optimal support for body activities

Operate via subconscious control

Have viscera as most of their effectors

ANS Versus Somatic Nervous System (SNS)

The ANS differs from the SNS in the following three areas

Effectors

Efferent pathways

Target organ responses

Effectors The effectors of the SNS are skeletal muscles

The effectors of the ANS are cardiac muscle, smooth muscle, and glands

Divisions of the ANS

The two divisions of the ANS are the sympathetic and parasympathetic

The sympathetic mobilizes the body during extreme situations

The parasympathetic performs maintenance activities and conserves body energy

The two divisions counterbalance each other’s activity

Anatomy of ANS

Efferent Pathways Heavily myelinated axons of the somatic motor

neurons extend from the CNS to the effector

Axons of the ANS are a two-neuron chain

The preganglionic (first) neuron has a lightly myelinated axon

The ganglionic (second) neuron extends to an effector organ

Neurotransmitter Effects

All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect

In the ANS:

Preganglionic fibers release ACh

Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory

ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

Comparison of Somatic and Autonomic Systems

Anatomy of ANS

Division Origin of Fibers Length of Fibers Location of Ganglia

Sympathetic Thoracolumbar region of the spinal cord

Short preganglionic and long postganglionic

Close to the spinal cord

Parasympathetic Brain and sacral spinal cord

Long preganglionic and short postganglionic

In the visceral effector organs

Role of the Parasympathetic Division

Concerned with keeping body energy use low

Involves the D activities – digestion, defecation, and diuresis

Its activity is illustrated in a person who relaxes after a meal

Blood pressure, heart rate, and respiratory rates are low

Gastrointestinal tract activity is high

The skin is warm and the pupils are constricted

Role of the Sympathetic Division

The sympathetic division is the “fight-or-flight” system

Involves E activities – exercise, excitement, emergency, and embarrassment

Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased

Its activity is illustrated by a person who is threatened

Heart rate increases, and breathing is rapid and deep

The skin is cold and sweaty, and the pupils dilate

Comparison of Somatic and Autonomic Systems

Anatomy of ANS

Parasympathetic Division Outflow

Cranial Outflow Cranial Nerve Ganglion Effector Organ(s)

Occulomotor (III) Ciliary Eye

Facial (VII) PterygopalatinSubmandibular

Salivary, nasal, and lacrimal glands

Glossopharyngeal (IX)

Otic Parotid salivary glands

Vagus (X) Located within the walls of target organs

Heart, lungs, and most visceral organs

Sacral Outflow S2-S4 Located within the walls of the target organs

Large intestine, urinary bladder, ureters, and reproductive organs

Parasympathetic Division Outflow

Sympathetic Outflow

Arises from spinal cord segments T1 through L2

Sympathetic neurons produce the lateral horns of the spinal cord

Preganglionic fibers pass through the white rami communicantes and synapse in the chain (paravertebral) ganglia

Fibers from T5-L2 form splanchnic nerves and synapse with collateral ganglia

Postganglionic fibers innervate the numerous organs of the body

Sympathetic Outflow

Sympathetic Tone

The sympathetic division controls blood pressure and keeps the blood vessels in a continual state of partial constriction

This sympathetic tone (vasomotor tone):

Constricts blood vessels and causes blood pressure to rise as needed

Prompts vessels to dilate if blood pressure is to be decreased

Parasympathetic Tone Parasympathetic tone:

Slows the heart

Dictates normal activity levels of the digestive and urinary systems

The sympathetic division can override these effects during times of stress

Drugs that block parasympathetic responses increase heart rate and block fecal and urinary retention

Unique Roles of the Sympathetic Division Regulates many functions not subject to

parasympathetic influence

These include the activity of the adrenal medulla, sweat glands (heat loss and gain), arrector pili muscles, kidneys, and most blood vessels (metabolism and digestion)

Levels of ANS Control

Levels of ANS Control

The hypothalamus is the main integration center of ANS activity

Subconscious cerebral input via limbic lobe connections influences hypothalamic function

Other controls come from the cerebral cortex, the reticular formation, and the spinal cord